Reduction of L10 Ordering Temperature in Chemically Synthesized FePt Nanoparticles by Addition of Au

S. Kang, Z. Jia, D. E. Nikles, and J.W. Harrell

1-xPtx nanoparticles.”]Chemically synthesized FePt nanoparticles are of considerable interest because of their potential for ultra-high density magnetic recording media [1]. Monodisperse particles of diameter 3-4 nm with organic coatings can be synthesized that can self-assemble into ordered arrays. If the particles are transformed into the high-anisotropy L10 phase, they should have sufficient magnetic stability for long-term data storage. As a conventional medium, storage densities as high as 1 terabit/in2 have been predicted (using thermally assisted writing). If one bit per particle addressing could be achieved, then the potential storage density would be ~40 terabits/in2, which is about 1000 times greater current state-of-the-art media. Although these nanoparticle arrays have tremendous technological potential, there are considerable materials issues that must be addressed. One of these issues is the high annealing temperature required for chemical ordering, which is more than 500 oC in pure FePt nanoparticles. At these high temperatures, the organic surfactant coating on the nanoparticles breaks down and the particles aggregate. We have been studying ways of reducing the ordering temperature by adding metals to the nanoparticles, as has been done for sputtered films of FePt and CoPt. We have recently found that the addition of Au can reduce the ordering temperature by more than 150 oC. Figure 1 shows the coercivity of [FePt]1-xAux nanoparticles as a function of annealing temperature for increasing Au concentration. Additive Au promotes chemical ordering beginning at 300 oC (or less) and the effect increases with Au concentration up to 24%, the maximum amount thus far studied. The effect of Au is greater than the enhancement that we recently reported for additive Ag [2]. For both Au and Ag the mechanism appears to be the same. Additive Au (Ag) expands the lattice. Upon annealing the Au (Ag) leaves the particles, creating vacancies that increase the mobility of the Fe and Pt atoms.

1. S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287, 1989 (2000).
2. S. Kang, J.W. Harrell, D.E. Nikles, Nano Letters 2, 1033 (2002).